The present invention relates to a novel modacrylic synthetic fiber having a flame resistance and an excellent devitrification preventing property and a process for preparing the modacrylic fiber.
Acrylic fibers have various excellent chemical and physical properties such as dyeing ability, hand touchness and fastness to light. In recent years, a demand for antiflaming of textile goods is rapidly increasing, and in particular, the flammability of acrylic fibers is in significant question. Acrylic fibers are essentially lacking in flame resistance, and accordingly are unsuitable for use in interior goods requiring antiflammability such as curtain and carpet, and clothes for babies, children and the old.
In order to improve this defect, there have hitherto been proposed various processes, e.g. a process in which a flame proofing agent is added to a spinning solution and the spinning is conducted, a process in which a flame proofing agent is incorporated into fibers by post-treatment, and a process in which a mixture of a flame resisting polymer and an acrylic polymer is extruded into a coagulation bath. However, these processes are not satisfactory for the reason that necessity of the use of large amounts of a flame proofing agent decreases the physical properties and hand touchness of acrylic fibers, or the flame resistance decreases due to elution of a flame proofing agent in dyeing and washing, or industrial production by a mix spinning process is difficult due to difficulty in maintining a mixed spinning solution uniform and stable.
On the other hand, as a process for semipermanently imparting flame resistance to acrylic fibers, there has been carried out a process in which acrylonitrile is copolymerized with a halogen-containing monomer such as vinyl chloride, vinylidene chloride, vinyl bromide or vinylidene bromide. In that case, the larger the content of a halogen in the obtained copolymer, the better the flame resistance. However, the process has a fatal defect that the devitrification preventing property is remarkably lowered with increasing the halogen content, whereby physical properties, processing property and commercial value of the fibers are impaired. The reason is considered to be that since acrylic fibers containing a large amount of halogenated vinyl monomer units are generally prepared by a wet spinning process, fine voids are easy to be formed in fiber structure, and moreover since the softening point of fibers is low, the fibers are easy to swell in a relatively low temperature range. For such reasons, it is particularly important to prevent the formation of fine voids in fiber structure in a coagulation bath in order to improve the devitrification property.
In case of the so-called acrylic synthetic fibers containing not less than 85% by weight of acrylonitrile, prevention of the devitrification has been generally conducted by copolymerizing with a sulfo group-containing vinyl monomer which also provides the fibers with dyeing sites. However, in case of the so-called modacrylic synthetic fibers which contain a large amount of halogenated vinyl monomer units, it is essentially difficult to form a dense fiber structure and, therefore, not only prevention of the devitrification is very difficult even if a sulfo group-containing vinyl monomer is incorporated, but also the incorporation of a sulfo group-containing vinyl monomer may rather bring about further decrease of devitrification preventing property. For such a reason, modacrylic fibers having both the satisfactory flame resistance and the devitrification preventing property have not yet been obtained. It is of urgent necessity for the fiber and textile industry to raise the devitrification preventing property of modacrylic fibers to the degree of the acrylic fibers.
Improvement of the devitrification of modacrylic fibers from the polymerization side is disclosed in Japanese Patent Publication Nos. 9299/1978 and 9300/1978, in which an acrylonitrile copolymer containing anionic monomer units in large quantities is added to the polymerization system and the polymerization is conducted in the presence of such a hydrophilic copolymer in a solution polymerization manner. However, these processes are not industrially advantageous. That is to say, since in these processes the hydrophilic copolymer must be previously prepared and added to the polymerization system before starting the solution polymerization, it is necessary to separately prepare the hydrophilic copolymer and to adjust the amount of the hydrophilic copolymer added depending on the polymerization yield in order to obtain stable properties of the fibers. Therefore, the procedure is very troublesome. Also, the obtained spinning solution is hard to maintain in a uniform solution, since the difference in the hydrophilic and hydrophobic properties of the mixed copolymers is relatively large. In case of using an acrylonitrile copolymer having a large hydrophilic property, there are cases where the hydrophilic copolymer is not dissolved or merely swelled depending on the kinds of the organic solvents used. The use of such a spinning solution lacking in uniformity causes troubles in spinning such as choking of nozzle and breaking of spinning fiber, and accordingly stable preparation of fibers is difficult. Furthermore, since the copolymer having a large hydrophilic property is easy to elute into a coagulation bath and a washing bath, the devitrification preventing effect is decreased and also fibers having a good dyeing ability is hard to stably obtain.
Also, in general, a solution polymerization process has a tendency that an average degree of polymerization of the produced polymer is easy to lower due to occurrence of chain transfer reaction between a solvent molecule and a propagating high molecule radical depending on the used solvent. Therefore, the physical properties and devitrification preventing property of fibers are hard to maintain. Further, a solution polymerization process has the defect that because of slow polymerization rate, the polymerization time is long and the reaction mixture is easy to be colored. These tendencies are particularly remarkable in the preparation of acrylonitrile copolymers containing vinyl chloride. For such a reason, adoptation of a solution polymerization is relatively rare in the preparation of acrylonitrile copolymers containing vinyl chloride among acrylonitrile copolymers having a relatively low acrylonitrile content used as raw materials of modacrylic fibers.
It is an object of the present invention to provide a modacrylic fiber having excellent flame resistance and devitrification preventing property.
A further object of the invention is to provide a process for preparing a modacrylic fiber having excellent flame resistance and devitrification preventing property.
These and other objects of the present invention will become apparent from the description hereinafter.